There are many designs for biomedical electrodes. Typically, these electrodes include a conductive adhesive hydrogel, which is in contact with a patient's skin, a conductive material in contact with the hydrogel so as to apply a uniform voltage or current to the gel, and a wire from the conductive material to a voltage source. Additionally, the electrodes may be covered by a protective dielectric film, such as, for example, Vinyl, Polyethylene, Polystyrene and Polyester.
U.S. Patent Application Publication No. 20030134545 to McAdams teaches the use of a conductive silver ink coated on a thin substrate having a sheet resistivity of between 0.01 and 50 ohms/□ (ohms per square). The substrate can be a polyester film or other suitable film. According to McAdams, the conductive silver coating has an irregular surface with a 4 μm peak to trough height, which could cause hot spots due to non-uniform current distribution.
U.S. Pat. No. 5,038,796 to Axelgaard discloses a conductive element that uses a weave or a knit fabric, wherein strands within the weave include stainless steel wire having a diameter of 8 microns. The wires are spaced apart from one another using a non-conducting fiber. The resulting diamond pattern of the weave provides a conductive fabric having improved stretchability and conformity around and/or between body extremities. Conductive gel fills in the interstitial space and reduces hot spots.
U.S. Pat. No. 4,934,383 to Glumac discloses a vapor deposited conductive film on polyester film. More specifically, Glumac discloses an electrode that uses a combination of a polymer film and a conductive layer to provide equalized current distribution and homogeneous impedance over the stimulating surface of the electrode. The combination of the conductive layer and polymer film can either be laminated together or vapor deposited. This enables placement of an electrical stud anywhere on the conductive layer, thereby providing for equalized current distribution.
While the above cited art presents improvements for biomedical electrodes, they each suffer from a number of technical problems. For example, the use of silver ink as disclosed in McAdams is quite inefficient. The silver flakes carried in fluid binder or ink must cause electrical tunneling in a fairly thick “0.0003” coating to be conductive. Hence, dry ink would have only a tiny fraction of the bulk conductivity of silver metal. Further, the binder and solvents in the ink can outgas and interact with the conductive gel, and the ink is quite expensive.
With respect to the technique disclosed in Axelgaard, costs can be relatively high and the resulting fabric may suffer from relatively low conductivity and uniformity. Further, production problems can arise, wherein die-cutting blades are dulled over time from cutting through the stainless steel wires. Additionally, stainless steel and other metals have a coefficient of thermal expansion of 10×10−6/° F., while plastics and polymers have expansion coefficients 2-3 times greater than metals. Due to the significantly different coefficients of thermal expansion, bowing or curling of the electrode assembly may result under some ambient thermal excursions. Also, shipping and storage may cause some delamination, resulting in potential hot spots. Embedding the fabric between two layers of gel may alleviate the problem, but will further add to the complexity of the assembly.
It is possible to use metalized films, wherein a layer of conductive material can be electrolytically deposited on a polymeric film. However, since the film (polymer) acts as a barrier, only one side is coated because there is a dielectric non-conducting film. If both sides were coated, only one side would effectively contact the gel. In any case, both the ink-coated or metalized film tends to be stiff and inflexible compared to a thin fabric.
With respect to the teachings of Glumac, a thick conductivity layer (e.g., 100-1000 Angstroms) must be deposited in order to achieve good sheet conductivity. However, these thick coatings can scratch and easily degrade, resulting in only one side being in contact with the gel.
In order to avoid hot spots (e.g., non-uniform distribution of current or voltage to the patient's skin in the area under the electrode), it is desirable to have a contacting conductive layer next to the gel that has a high conductivity. This material should be compatible with the gel, have sufficient surface area to provide good adhesive contact with the gel, be thin, flexible, stretchable, rugged and conform to body shapes, yet be easily processed, die cut and low in cost.